Continuous preparation of fermented milk products

ABSTRACT

Yogurt and other fermented milk products are prepared in a continuous process in which a continuous stream of milk is added to a fermenting mixture of milk and a starter in a prefermenting tank. The pH of the mixture in the prefermenting tank is maintained above the value at which stirring of the mixture will cause syneresis to occur in the final product. A portion of the prefermented mixture is sprayed into a coagulating tank in which further fermentation and coagulation take place to form a gel structure. In the coagulating tank, the mixture, when its pH surpasses the acidity at which stirring will no longer cause syneresis, is subjected to shearing forces by stirring the gel structure near the bottom of the tank to form a smooth consistency without disturbing that part of the gel structure upstream in the coagulating tank in which sufficient acidity has not been produced and thereafter filling into containers the product having a smooth consistency through a discharge pipe in the bottom of the coagulating tank.

'United States Patent 1 Driessen et al.

[ Dec. 2, 1975 l CONTINUOUS PREPARATION OF FERMENTED MILK PRODUCTS [75] Inventors: Fransiscus M. Driessen, Ede;

Jacobus J. Stadhouders, Bennekom; Jacob Ubbels, Ede, all of Netherlands [73] Assignee: Stichting Bedrijven Van Het Nederlands Instituut Voor Zuivelonderzoek, Ede, Netherlands 221 Filed: Apr. 9, 1973 211 Appl. No.: 349,196

[30] Foreign Application Priority Data Apr. 7, 1972 Netherlands 7204690 [52] US. Cl. 426/43; 99/452; 99/460; 195/96; 426/34; 426/61 [51] Int. Cl A23c 9/12; A230 17/00 [58] Field of Search 426/34, 39, 43, 61, 185, 426/356; 195/96; 99/452, 460

[56] References Cited UNITED STATES PATENTS 2,908,575 10/1959 Spiess, Jr. et al. 426/39 X 3,432,306 3/1969 Edwards 426/356 FOREIGN PATENTS OR APPLICATIONS 161,169 2/1955 Australia Primary Examiner-David M. Naff Attorney, Agent, or Firm-Fleit & Jacobson [57] ABSTRACT Yogurt and other fermented milk products are prepared in a continuous process in which a continuous stream of milk is added to a fermenting mixture of milk and a starter in a prefermenting tank. The pH of the mixture in the prefermenting tank is maintained above the value at which stirring of the mixture will cause syneresis to occur in the final product. A portion of the prefermented mixture is sprayed into a coagulating tank in which further fermentation and coagulation take place to form a gel structure. 1n the coagulating tank, the mixture, when its pH surpasses the acidity at which stirring will no longer cause syneresis, is subjected to shearing forces by stirring the gel structure near the bottom of the tank to form a smooth consistency without disturbing that part of the gel structure upstream in the coagulating tank in which sufficient acidity has not been produced and thereafter filling into containers the product having a smooth consistency through a discharge pipe in the bottom of the coagulating tank.

23 Claims, 4 Drawing Figures US. Patent Dec. 2, 1975 Shfifit 1 014 3,924,007

US. Patent Dec. 2, 1975 Sheet4 (51 4 3,924,007

Amm EV wE: coa nau A wn u m n wow 3 tame cm mm n B 25 c mELmm CONTINUOUS PREPARATION OF FERMENTED MILK PRODUCTS The present invention relates to a process for the continuous preparation of yogurt and Bulgarian yogurt, by adding a continuous stream of milk to a fermenting mixture of milk and a starter in a pre-fermenting tank, while maintaining the temperature at 4248C and a pH not below about 5.3, passing a similar quantity of the fermenting product into a coagulating tank, in which the further fermentation and coagulation can take place.

The present invention relates also to a process for the continuous preparation of cultured buttermilk and lactic starters by adding a continuous stream of milk to a fermenting mixture of milk and a starter in a pre-fermenting tank, while maintaining the temperature at 20-35C and the pH not below about 5.3, passing a similar quantity of the fermenting product into a coagulating tank in which the further fermentation and coagulation can take place.

Such a semi-continuous process is known for the publication of Girginovs article in Die Lebensmittelindustrie, 12, 263(1965) whereby 1-3% by weight of a fresh uncooled culture of Lactobacillus bulgaricus and Streptococcus thermophilus having an acidity of 30-32Sl-l (-80N) is added to pasteurized and homogenized milk, after which the mixture is allowed to ferment at a temperature of 4648C until an acidity is reached of approximately 30N; subsequently the continuous pre-fermentation process starts, during which the quantities of pasteurized, non-fermented milk having a temperature of 46-48C added thereto are equal to the quantities of fermented product simultaneously discharged, and whereby the acidity of 9.5l2Sl-l (-30N) and the level of liquid in the tank are maintained. The partly fermented milk is then cooled to a temperature of 3233C and packed in containers, in which further fermentation takes place at the same temperature until the desired acidity is reached. Finally, the product is cooled to a temperature of 5-6C. Girginovs process is a semi-continuous process.

It was found now that yogurt may be prepared in a continuous process by allowing the fermenting mixture to pass in the form of a plug stream through a coagulating tank, which is provided with a discharge pipe at the bottom side, whereby the gel structure of that part of the plug stream, which has a pH not exceeding 4.7 and a temperature of from 33 to 37C, is disturbed by subjecting it to shearing forces, and subsequently filling containers with the product prepared.

Further it was found now, that Bulgarian yogurt may be prepared in a continuous process by allowing the fermenting mixture to pass in the form of a plug stream through a coagulating tank, which is provided with a discharge pipe at the bottom side, whereby the gel structure of that part of the plug stream, which has a pH not exceeding 5.1 and a temperature of from 33 to 37C, is disturbed by subjecting it to shearing forces, and subsequently filling containers with the product prepared.

Likewise, it was found now that cultured buttermilk and lactic starter may be prepared in a continuous process by allowing the fermenting mixture to pass in the form of a plug stream through a coagulating tank, which is provided with a discharge pipe at the bottom side, whereby the gel structure of that part of the plug stream, which has a pH not exceeding 4.7, is disturbed by subjecting it to shearing forces, and subsequently filling containers with the product prepared.

Preferably a coagulating tank is used, the wall of which is subjected to a pre-treatment; for example, a pre-treatment of a stainless steel tank with a lecithin emulsion, lecithin, or similar surface active substances. The use of PTFE is also efficient.

The fermenting mixture should be passed through a coagulating tank in the form of a plug stream because the structure of the product would be detrimentally affected if deliberately produced turbulences, or unintentionally generated turbulences, should occur in such a mixture, causing a.o. syneresis to occur. In this connection it is desirable to cool the pre-fermented milk to a temperature of from 31 to 37C (see Th.E.Galesloot, Off.Org. FNZ 47,720 (1955)) because in said temperature range sufficient quantities of mucilageous substances are formed to provide a yogurt having after stirring a sufficient viscosity and moreover than the syneresis will be less pronounced than on omitting the cooling of the pre-fermented milk. If the fermenting milk has obtained a sufficient acidity, i.e. if the acidity reached is between and N, stirring may be continued without causing permanent damage to the structure and/ or whey separation. It is to be understood that the final acidity is also determined by the protein content of the milk.

It is preferable to enable the plug stream to be formed by spraying the prefermented mixture into a coagulating tank, the walls of which are coated with a surface active compound. For practical reasons it is of importance that the plug stream in the coagulating tank is passed at a rate of speed of at most 5 cm/minute at a temperature of 37C.

In order to obtain a product which is as homogeneous as is possible, it is desirable to hold the temperature at which the fermenting mixture is passed into the coagulating tank at a constant value. Although the viscosity of the homogeneous product obtained by disturbing the gel structure of yogurt by stirring is not very critical, it is preferable to disturb the gel structure no longer than is necessary for the viscosity of the homogeneous product (measured according to G. Posthumus Off.Org. FNZ 46,55 (1954)), 20-40 seconds. Preferably the fermented product is cooled to a temperature of from 5 to 6C between the coagulating tank and the filling apparatus.

Whereas it is obviously desirable, preceding to the filling operation, to disturb in the lower part of the coagulating tank the structure of the fermented milk, changing it from a firm gel into a smooth highly-viscous fluid, it should be prevented by all means that the structure of the fermented milk is disturbed at an upstream level at which the pH has not yet decreased to the extent that a disturbance will not cause ultimately a serum expulsion. To this end, for example, a plate having the dimensions of a plane cross-section of the tank, and adapted to be moved parallel to the axis of said tank, may be provided with a number of perforations distributed across the surface and may be lowered following the movement of the fermenting milk at substantially the same rate of speed, in the same direction over some distance, to be subsequently moved in the opposite direction at a much faster speed, over a distance that is not so great as to cause the plate to reach that part of the tank in which fermentation has not yet proceeded to a sufficient extent to allow stirring without any permanent adverse effects on the structure. An

alternative way of stirring may consist in that the plug stream on its slow passage downstream of the zone where a sufficient acidity has been reached to allow stirring without any permanent adverse effects on the structure is temporarily interrupted by a partition plate, for example, by pushing through a plate which is provided with an aperture having the same shape as the cross-section of the tank, after which the structure of the fermented milk under said plate can be disturbed by stirring. If the plate is returned to its original position, after the stirring process has been stopped, the plug stream can resume its slow passage, so that the stirred, fermented milk is pressed out of the tank. Other methods for stirring the fermented milk in the tank without disturbing the structure of the upstream milk fall also under the protective scope of the process according to the present invention.

The moment at which the structure in the column can be disturbed according to any one of these methods can be controlled by a measuring of the pH in the fermenting milk.

A continuous process for the fennentation of milk products according to the present invention can simply be controlled by means of remote control, which leads to a uniform final acidity. The viscosity of the filled product may be affected by disturbing the structure to a greater or lesser extent, e.g. by stirring in a less intensive manner or by displacing the perforated plate against the direction of the flow of milk at a different rate of speed. Furthermore the required daily output can be spread out over a longer period of time, making it possible for the bottled product to be holder pasteurized in a simple manner as filling occurs at a low speed. The keeping qualities can thus be improved to a considerable extent. In addition to this it becomes very much simpler to apply an aseptic method owing to the low bottling speed and owing to the fact that the preparation of the product takes place in a closed apparatus.

The present invention may be more readily understood by reference to the following drawings wherein:

FIG. 1 is a view partly in cross section and partly schematic of the apparatus used to carry out the inventive process.

FIG. 2 is a graph illustrating the fermentation rate of yogurt as a function of temperature.

FIG. 3 is a graph similar to FIG. 2 illustrating the fermentation rate of cultured buttermilk as a function of temperature.

FIG. 4 is a graph similar to FIGS. 2 and 3 illustrating the fermentation rate of Bulgarian yogurt as a function of temperature; and

FIG. 5 is a view of an apparatus used in accordance with the present invention for determining conditions of use for different perforated plates in the coagulating tank of the inventive apparatus of FIG. 1.

The process of the present invention can be carried out with the aid of an apparatus, which is diagrammatically shown in FIG. 1. Storage tank 1 is connected with prefermenting tank 6 by means of a pipeline through pump 2. Pump 2 is activated by a switch 3 which in its turn is controlled by a signal from the pH meter 4. Tank 6 is equipped with an overflow 9 for the discharge of pre-fermented product, a stirrer 7 and a temperature sensor 8, a pH sensor 5 and, for example, a device for spraying hot water against the wall of the tank. Overflow 9 is directly connected with cooler 10, which ends in a distributing device 11 at the top portion of coagulating tank 12.

The coagulating tank 12 is preferably constructed with double walls, while it is provided with a device to hold the temperature of the contents at a constant value. The distributing device 11 may, for instance, comprise a conical disc which is rotating at high speed and with which the pre-fermented product which flows from cooler 10 onto said disc is atomized.

At the lower side of the coagulating tank 12 is mounted a discharge pipe which is connected with a pump 13 with which the final product is pumped to a filling apparatus. Said pump 13 can be controlled by the switch 3, provided its capacity is equal to that of pump 2. Otherwise it is possible to use a similar control as is used to activate the pump 13 if the pH at the bottom of the vat 12 has reached a predetermined value, whereby the capacity of pump 13 should be somewhat larger than that of pump 2, or should be equal thereto.

In carrying out the process in the apparatus described above, the wall of the coagulating tank 12 is preferably pretreated with a surface active compound. For this purpose lecithin, a lecithin emulsion, a Teepol solution, or any other surface active substance can be used, for instance, for a stainless steel wall (see A.G.J. Arentzen, Off.Org. FNZ 58,479 (1966)). The wall may also be coated with polytetrafluoroethylene.

The storage tank contains, if required, homogenized and standardized, pasteurized or sterilized milk. Said milk, in dependency on the product to be prepared, has a temperature of from 42 to 48C, where yogurt is concerned, and a temperature of from 20 to 35C where starters or cultured buttermilk are to be prepared. The milk is pumped into the prefermenting tank 6, after which l3% by volume of the fermentation culture is added thereto.

As soon as the milk in said tank has reached a stage in the fermentation process in which the pH value is 5.3 to 5.8, in dependency on the fermentation temperature, the pump 2 is activated by means of the pH meter 4 and the switch 3. This arranges for milk to be pumped into the pre-fermenting tank, while a similar quantity of the pre-fermented milk passes through the overflow 9 and the cooler 10 to the distributing device 11 in the upright coagulating tank 12. If owing to this inflow of unfermented milk, the pH reaches a value which is, for instance, O.l of a pH unit higher than the adjusted pH value, the pump 2 is switched off again.

The pre-fermented milk can be cooled from the fermentation temperature to a temperature of from 33 to 37C by means of the cooler 10. This is to be recommended if yogurt is prepared, but not if starters or cultured buttermilk and similar products are prepared.

Before the final product is removed by pumping first of all the structure of the same is stirred into a smooth consistency. This is only carried out in the bottom part of the coagulating tankwherein the acidity reached is already sufficiently high to avoid any permanent adverse effects on the structure, while it has to be prevented that the structure upstream of said part is also disturbed. To this end the vat has been equipped with an appropriate stirring means, for instance the means 14. Preferably said means comprises a movable plate which is provided with perforations uniformly spaced over its entire surface, the area of those perforations amounting to at most 20% of the total area of the plate.

To start the process the space under the stirring means 14 is filledwith stirred final product. The distributing device is thereupon lowered to just above the stirring means 14.

Subsequently the 'pre fermented product is allowed to start filling the coagulating tank 12, which product flows from the pre-fermenting tank 6. During the filling process the distributing device 11 is gradually moved in an upward direction, so that under all conditions prefermented milk is sprayed over the surface of the milk in the coagulating tank 12.

During the process of filling of the coagulating tank 12 pump 13 remains inactivated. The moment at which 5 tables is also stated in which pH range the culture cannot be agitated without causing permanent damage to the structure.

The dimensions of the apparatus for a desired capacity of the same can be determined from the aforementhe discharge of the final product can be started with tioned data. An example thereof will be given hereinafthe aid of said pump is, for instance, determined with a pH sensor which is mounted through the wall of the coagulating tank 12 in a zone which is just above the part with the fermenting milk, the structure of which is to be disturbed just before the filling or bottling process. An alternative possibility is to arrange that the activation of the pump 13 is controlled by the pH sensor 15 in conjunction with a device appropriate to the purpose. A similar control is also possible by means of a level sensor 16. Another possibility is to determine the moment at which the correct acidity or pH is reached in a sample of the pre-fermented milk, which is taken when the first quantity of pre-ferrnented milk is passed into the coagulating tank and maintained at the temperature of fermentation. Finally, it is possible to start the pump at a predetermined time to be counted from the moment when filling of the coagulating tank was started, provided the fermentation rate in the prefermenting tank has not shown any deviations from the usual rate.

The content of the apparatus depends of course on the required hourly capacity. However, a predeterter. It it is desired per hour to ferment 500 l of yogurt of 80N at a temperature of 45C, starting from milk having an acidity of 15N, while drawing off will occur at an acidity of 32N, at which the yogurt will be passed 15 into a coagulating tank, the flow rate in the fermenting tank will be determined by assuming that the rise in acidity by the proceeding fermentation process period of time by the addition of non-fermented milk addition. Thus fermentation 0.67 addition from which it follows that the average residence time of the milk in the fermenting tank t 25.4 minutes.

In an hours time the content of the tank are thus 60/25 .4 2.36 times changed and for an hourly capacmined ratio between the content of the prefermenting 40 ity of 500 l the tank should thus have a capacity of tank 6 and of the coagulating tank 12 is inperative, which ratio can be deduced from the fermentation rates at the temperature prevailing in each of said tanks. The connection between the reached acidity and the incubation period for a yogurt culture is shown in FIG. 2. The following fermentation rates were determined for the linear part of the curves found, by applying the method of the least squares; at a temperature of 45C a 0.67 t 32.17

37C a 0.50 t 31.17

36C 2 a 0.45 t 29.86

C a 0.40 t 26.71

34C a 0.38 t 29.56 in which 1 stands for the incubation period in minutes and a stands for acidity in N reached after said period.

The curves of FIG. 3 were found for a lactic starter culture or cultured buttermilk culture and from these were determined the following fermentation rates; at a temperature of 30C a 0.34 t- 37.49

25C a 0.29 t- 43.66

The curves found in FIG. 4 are those found for Bulgarian yogurt and the fennentation rate found at a temperature of 45C a= 1.14 t 80.00

40C a 0.79 t-- 76.71

37C a 0.59 t- 6404 34C a 0.56 t 87.66

If the fermentation process is continued in the coagulating tank at a temperature of 37C, the residence time t of the milk in said tank may be calculated from 0.50

t= 80 32 or t= 48/050 96 minutes, if the acidity of 32N should increase to 80N.

The capacity of the tank is thus 96/60 X 500 800 1. It is not desirable to increase the cross-section of the tank too much. In order to damage the structure prior 50 to drawing off the final product the stirring means 14 has to be raised at a rapid rate of speed. The force required thereto increases in proportion to the area of said stirring means. On the other hand the area must not be made too small, as this would result in the height of the coagulating tank increasing to a disproportionate extent. Apart from possible difficulties arising from mounting a very high tank, this causes the rate of speed at which the fermenting milk flows along the wall of the tank to become too fast. It has been determined at various temperatures to which extent the structure of the fermenting milk was permanently damaged if the said milk was made to flow along a stainless steel wall of the tank, treated with lecithin, at a rate of speed of 5 cm/minute. Some results have been given in table D,

from which it appears that at a temperature at which on the one hand the fonnation of mucilage is sufficiently large 3 8C) and on the other hand the fermentation rate is not too low 33C) a rate of not over 5 cm/minute with respect to the wall' is just permissible. In the example given the milk, at a temperature of 37C, may be displaced along the wall over a distance of at most 480 cm in 96 minutes.

This entails that the diameter of the tank should be at least A tank constructed with a height of two to four meter is preferably used.

The stirring means 14 can, for instance, consist of a flat plate which fits closely in the coagulating tank by means of a sealing ring and is adapted to be moved in an upward and downward direction so that it moves parallel to the axial direction of said tank.

The downward movement should be performed at substantially the same speed at which the fermenting milk passes along the wall of the container. During the upward movement the structure of the final product has to be disturbed and for this purpose the plate has to be moved at a far greater speed. During this movement the product is forced to pass through a number of perforations provided in the plate with the result that there is obtained a smooth structure, while the viscosity does not decrease to too great an extent. The rate at which such a plate has to be raised in order to obtain a smooth structure is determined by the diameter of the perforations and their number per unit area. The rate must not be too great, as in that instance the viscosity of the yogurt is reduced too much, but the rate has to be sufficiently great to obtain a smooth structure.

2 Liter of yogurt were prepared in a cylindrical tank having a diameter of 12.5 cm and, diagramatically shown in FIG. 5, at a temperature of 37C. In the Figure 1 stands for the fermenting tank, 2 stands for a support from which the tank 1 is suspended by means of clamping means 3, 4 stands for a manometer, 5 stands for a pipeline through which nitrogen is pressed into said tank, 6 stands for a sealing ring, 7 stands for the stirring plate to be tested. After having reached an acidity of 80N the yogurt, with nitrogen of a pressure known as such, was pressed through the perforated plate in a given period of time. The viscosity of the yogurt flown out of the tank was measured and the structure thereof was examined. The results thus obtained are given in Table E. Five different plates a, b, c, d and e were used for this test. The plates a, b and c were constructed with perforations having a diameter of 1.0, 0.36 and 0.25 cm respectively. Plate a had been constructed with one perforation only. The number of perforations for the plates b and c were defined by calculation, so that with a uniform outflow capacity the average shearing rate in each perforation of the three plates would be the same.

Perforations having a sectional size of 0.15 cm were provided in the plate d in the same points as in plate 0. In these perforations the average shearing rate is thus greater than in the plates a, b and c, while processing conditions were the same. The perforations were provided such that they were uniformly spaced across the surface of each plate. Perforations having a sectional size of 0.25 cm are provided in the plate such that they are uniformly spaced at the angles of equilateral triangles having a size of 0.75 cm. 10% of the surface of said plate approximately is open as a result thereof. The following directives are true for the calculation:

(for plate a E= 637 cm s) 7 the average shearing rate (s) c a constant value.

From the data for plate a it can be calculated that C 637. With this value as base it follows for the plates b and c that n,, 22 for d 0.36 cm, and n 62 for d 0.25 cm, if Vl: 500 em /s remains constant.

From the data of Table E it appears that with a similar average shearing rate y the viscosity still varies with the plates a, b and c. This may partly be attributed to the fact that as a result of the varying number of perforations per unit area in the three stirring plates, the displacement of the yogurt over said plates is accompanied with a variation in shearing rate.

The process will now be explained in more detail with reference to a number of examples.

EXAMPLE I.

Standardized (2.95% fat), homogenized (150 atm. at 55C), and pasteurized (5 minutes at C) milk having a temperature of 45C was introduced in a pre-fermenting tank having a content of 12 1 until the tank was nearly completely filled. To this milk was subsequently added /2 l. of a yogurt culture consisting of S. thermophilus and L. bulgaricus. The stirring device was activated and the temperature held at 45C. When the pH had been reduced to a value of 5.70 after minutes, the pH was held at a value of between 5.70 and 5.72 with the aid of a pH adjusting means by pumping milk in the bottom part of the fermenting tank. The volumetric rate of flow amounted to 24 l/h. An equal amount of pre-fermented milk flowed simultaneously through the overflow 9 out of the tank and through a cooler, where the temperature was reduced to 37C, to

the rapidly rotating distributing device which had been positioned in its lowest position.

The coagulating tank was made of stainless steel and had been constructed with double walls. The internal diameter amounted to 35.7 cm, and the height to 80 cm. In the lower portion of said tank was mounted a stirring means, comprising a circular plate of stainless steel provided with perforations of 0.25 cm diameter at a mutual distance from one another, which distance was 1.5 cm. This plate was adapted to be moved in an upward and downward direction over a distance of 10 cm with the aid of a lever mechanism. The space under said plate was previously filled with stirred yogurt. The

stirring plate and the walls of the tank were treated with A commercial soy bean lecithin. Subsequently the distributing device was raised at the same rate of speed as the rate of speed at which the liquid level rose in the container. From the very moment at which the acidity of the yogurt, just above the stirring plate, had reached a value of 90N, the drawing off process was started with a volumetric rate of speed of 24 l/h. For this purpose the stirring plate was raised at a rate of speed of 3 cm/s. and was subsequently lowered again in 25 minutes. The stirred yogurt was pumped out of the tank with the aid of a metering pump, the pump operating synchronously with the milk pump. In two hours time the acidity of the yogurt flowing out of the tank showed values between 85 and 95N, while the viscosity, measured with the Posthumus cup (Off. Org. FNZ, 46, 55 (1954)) varied between 30 and 45 seconds at C. The quality of the yogurt was excellent.

EXAMPLE II.

A Bulgarian yogurt was prepared, starting with conventional yogurt milk, from which Vs of the volume was removed by evaporation. The procedure set forth in example I was repeated in full, on the understanding that the pH value at which evaporated milk began to be pumped into the tank amounted to 5.8, and the acidity at which the yogurt began to be pumped out of the tank amounted to 100N.

skimmed milk containing 0.4% by weight of fat was used, having a temperature of 30C, which temperature was also maintained in the fermenting tank. After this tank had been filled, the milk was inoculated with l /2% of a BD starter. After 225 minutes a pH value had been reached of 5.35. Then the preferrnented milk was pumped into the fermenting tank, during which process the pre-fermented milk was not allowed to cool. Subsequently milk was pumped into the tank at a rate of speed amounting to 8 l/h. The space under the stirring plate of the coagulating tank was in this case filled with buttermilk. Inasmuch as the thickness of the layer of liquid in the tank only amounted to 12 cm because of the size of the volumetric flow (8 l/h) and the fermentation rate of speed (from N to N in minutes), the distance over which the stirring plate was displaced was reduced to l to 2 cm. During 90 minutes there was thus obtained a cultured buttermilk having a most satisfactory structure, the acidity of which appeared to be 85N continually.

EXAMPLE IV A starter was prepared in a manner corresponding with the manner described in Example III, except for the fact that a fermenting temperature was used of 25C, while furthermore the starting material used was skimmed milk containing 0.1% by weight of fat. The fmal product thus obtained was used to be added as a starter to cheese milk. No deviations of the normal pro- TABLE A Dependence of structure in the final product on disturbance at various acidity during fermentation of normal yogurt Solids non-fat content 8% incubation period Inoculation 2 /2% (v/v) EXAMPLE III Using the same apparatus as is used in Example I, cultured buttermilk was prepared for which partly cedure were observed in the preparation of cheese, while the cheese thus obtained had a satisfactory quality.

TABLE B Solids-non-fat content 8% Dependence of structure in the final product on disturbance at various acidity during fermentation of cultured buttermilk Fat con tent 0.4%

BD-starter Inoculation l /2% (v/v) Incubation incubation period Acidity appearance after temperature prior to agitating complete incubation (C) (minutes) (N) (pH) 155 28 5.99 satisfactory I 75 32 5.81 satisfactory 30 I 35 5.67 satisfactory 225 44 5.35 satisfactory 255 58 5.07 very much whey 285 72 4.90 some whey TABLE c Dependence of structure in the final produce on disturbance at various acidity during fermentation of Bulgarian yogurt Solids non-fat content 13.6% Inoculation 2/2% (v/v) Incubation Incubation period Acidity Aftcr complete incubation temperature prior to (N) (pH) appearance viscosity (C) agitating (s) 90 6. l 9 satisfactory 74 100 31 6.17 satisfactory 73 1 10 34 6.07 satisfactory 67 125 37 6.04 satisfactory 69 145 41 5.86 satisfactory 72 180 67 5.24 some whey 59 210 93 4.95 some whey 1 15 29 6.21 satisfactory 68 125 31 6.20 satisfactory 72 140 3 2 6.15 satisfactory 69 37 I55 36 6.09 satisfactory 170 41 5.94 satisfactory 185 45 5 .78 satisfactory 72 215 60 5.36 some whey 52 245 82 5.14 some whey 54 260 87 500 some whey 49 135 29 6.29 satisfactory 77 I60 31 6.23 satisfactory 175 33 6.15 satisfactory 88 34 205 38.5 6.08 satisfactory 220 43 6.00 satisfactory 87 235 45 5.84 satisfactory 82 250 48 5.70 satisfactory 89 280 67 5.34 some whey 70 295 80 520 some whey 69 TABLE D Feasible rate of yogurt movement along the wall of the coagulation tank during the critical state of fermentation temperature speed along range final appearance viscosity (C) the wall start finish acidity (s) (cm/minute) (N) (N) 37 5 33-39 81 fair 32 36 5 37-44 81 fair 29 35 5 38-44 80 satisfactory 56 34 5 42-48 83 satisfactory 49 34.6 10 44-47 96 curdled 37 37 4 42-52 85 fair TABLE E Effect of shearing on quality of yogurt (for description of experiments with apparatus of FIG. 5, vide text) pressure time of viscosity diameter (kgf/cm outflow (Posthumus cup) appearance perforations (s) (5) (scale 3-8) (cm) What is claimed is:

1. A process for the continuous preparation of yoghurt comprising adding a continuous stream of milk to a fermenting mixture consisting of milk and starter in a prefermenting tank while maintaining the temperature of the mixture at 4248C, transferring a similar quantity of the fermenting mixture which has a pH not below about 5.3 into a coagulating tank in which further fermentation and coagulation take place, cooling the fermenting mixture during passage into the coagulating tank, spraying the fermenting mixture into the coagulating tank, allowing the fermenting mixture to flow downward through the coagulating tank and coagulate and form a gel structure, and near the bottom of the tank where the acidity is sufficient to produce a pH of the gel structure of about 4.7 or below and the acidity is sufficiently high to prevent syneresis by stirring, stirring the gel structure near the bottom of the tank to form a smooth consistency without disturbing that part of the gel structure upstream in the coagulating tank in which sufficient acidity has not been produced, and subsequently filling containers with the resulting product through a discharge pipe in the bottom of the coag-.

ulating tank.

2. A process according to claim 1 in which the coagulating tank walls are coated with a surface-active compound.

3. A process according to claim 1 in which the speed of the fermenting mixture in the coagulating tank is at most 5 cm/minute at a temperature of 37C.

4. A process according to claim 1 in which the temperature at which the fermenting mixture is transferred coagulating tank is held constant.

5. The process of claim 1 in which the temperature near the bottom of the tank is about 33 to 37C.

6. The process of claim 1 in which the pH of the fermenting mixture transferred into the coagulating tank is about 5.3 to 5.8.

7. The process of claim 1 in which the gel structure is stirred by moving a perforated partition plate upward in the coagulating tank while causing the gel structure to pass downward through the perforations in said perforated plate.

8. The process of claim 1 in which the gel structure is stirred by temporarily interrupting the downward flow of the fermenting mixture with a partition plate and stirring the gel structure below said partition plate.

9. A process for the continuous preparation of Bulgarian yoghurt comprising adding a continuous stream of milk to a fermenting mixture consisting of milk and starter in a prefermenting tank while maintaining the temperature of the mixture at 4248C, transferring a similar quantity of the fermenting mixture which has a pH not below about 5.3 into a coagulating tank in which further fermentation and coagulation take place, cooling the fermenting mixture during passage into the coagulating tank, spraying the fermenting mixture into the coagulating tank, allowing the fermenting mixture to flow downward through the coagulating tank and coagulate and form a gel structure, and near the bottom of the tank where the acidity is sufficient to produce a pH of the gel structure of about 5.1 or below and the acidity is sufficiently high to prevent syneresis by stirring, stirring the gel structure near the bottom of the tank to form a smooth consistency without disturbing that part of the gel structure upstream in the coagulating tank in which sufficient acidity has not been pro duced, and subsequently filling containers with the resulting product through a discharge pipe in the bottom of the coagulating tank.

10. A process according to claim 9 in which the coagulating tank walls are coated with a surfaceactive compound.

11. A process according to claim 9 in which the speed of the fermenting mixture in the coagulating tank is at most 5 cm/minute at a temperature of 37C.

12. A process according to claim 9 in which the temperature at which the fermenting mixture is transferred into the coagulating tank is held constant.

13. The process of claim 9 in which the temperature near the bottom of the tank is about 33 to 37C.

14. The process of claim 9 in which the pH of the fermenting mixture transferred into the coagulating tank is about 5.3 to 5.8. I

15. The process of claim 9 in which the gel structure is stirred by moving a perforated partition plate upward in the coagulating tank while causing the gel structure to pass downward through the perforations in said perforated plate.

16. The process of claim 9 in which the gel structure is stirred by temporarily interrupting the downward flow of the fermenting mixture with a partition plate and stirring the gel structure below said partition plate.

17. A process for the continuous preparation of cultured buttermilk or lactic starter comprising adding a continuous stream of milk to a fermenting mixture consisting of milk and starter in a pre-fermenting tank while maintaining the temperature of the mixture at 2035C, transferring a similar quantity of the fermenting mixture which has a pH not below about 5.3 into a coagulating tank in which further fermentation and coagulation take place, spraying the fermenting mixture into the coagulating tank, allowing the fermenting mixture to flow downward through the coagulating tank and coagulate and form a gel structure, and near the bottom of the tank where the acidity is sufficient to produce a pH of the gel structure of about 4.7 or below and the acidity is sufficiently high to prevent syneresis by stirring, stirring the gel structure near the bottom of the tank to form a smooth consistency without disturbing that part of the gel structure upstream in the coagulating tank in which sufficient acidity has not been produced, and subsequently filling containers with the resulting product through a discharge pipe in the bottom of the coagulating tank.

18. A process according to claim 17 in which the coagulating tank walls are coated with a surface active compound.

19. A process according to claim 17 in which the ferred into the coagulating tank is held constant.

21. The process of claim 17 in which the pH of the fermenting mixture transferred into the coagulating tank is about 5.3 to 5.8.

22. The process of claim 17 in which the gel structure s stirred by moving a perforated partition plate upward in the coagulating tank while causing the gel structure to pass downward through the perforations in said perforated plate.

23. The process of claim 17 in which the gel structure is stirred by temporarily interrupting the downward flow of the fermenting mixture with a partition plate and stirring the gel structure below said partition plate.

UNITED STATES PATENT OFFICE QERTIHCATE OF CORRECTION Patent No. 3924O07 Dated December 2, 1975 Fransiscus M. Driessen et al. Page 1 of 2 Inventor s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

FIGURE 1 should appear as shown on the attached sheet.

ESigncd and Sealed this twenty-fifth Day Of May 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oflan'nls and Trademarks Patent No. 3,924,007 page 2 of 2 

1. A PROCESS FOR THE CONTINOUS PREPARATION OF YOGHURT COMPRISING ADDING A CONTINUOUS STREAM OF MILK TO A FERMENTING MIXTURE CONSISTING OF MILK AND STARTER IN A PREFERMENTING TANK WHILE MAINTAINING THE TEMPERATURE OF THE MIXTURE AT 42*-48*C. TRANSFERRING A SIMILAR QUANTITY OF THE FERMENTING MIXTURE WHICH HAS A PH NOT BELOW ABOUT 5.3 INTO A COAGULATING TANK IN WHICH FURTHER FERMENTATION AND COAGULATION TAKE PLACE, COOLING THE FERMENTING MIXTURE DURING PASSAGE INTO THE COAGULATING TANK, SPRAYING THE FERMENTING MIXTURE INTO THE 0COAGULATING TANK, ALLOWING THE FERMENTING MIXTURE TO FLOW DOWNWARD THROUGH THE COAGULATING TANK AND COAGULATE AND FORM A GEL STRUCTURE, AND NEAR THE BOTTOM OF THE TANK WHERE THE ACIDITY IS SUFFICIENT TO PRODUCE A PH OF THE GEL STRUCTURE OF ABOUT 4.7 OR BELOW AND THE ACIDITY IS SUFFICIENTLY HIGH TO PREVENT SYNERESIS BY STIRRING, STIRRING THE GEL STRUCTURE NEAR THE BOTTOM OF THE TANK TO FORM A SMOOTHE CONSISTENCY WITHOUT DISTURBING THAT PART OF THE GEL STRUCTURE UPSTREAM IN THE COAGULATING TANK IN WHICH SUFFICIENT ACIDITY HAS NOT BEEN PRODUCED, AND SUBSEQUENTLY FILLING CONTAINERS WITH THE RESULTING PRODUCT THROUGH A DISCHARGE PIPE IN THE BOTTOM OF THE COAGULATING TANK.
 2. A process according to claim 1 in which the coagulating tank walls are coated with a surface-active compound.
 3. A process according to claim 1 in which the speed of the fermenting mixture in the coagulating tank is at most 5 cm/minute at a temperature of 37*C.
 4. A process according to claim 1 in which the temperature at which the fermenting mixture is transferred coagulating tank is held constant.
 5. The process of claim 1 in which the temperature near the bottom of the tank is about 33* to 37*C.
 6. The process of claim 1 in which the pH of the fermenting mixture transferred into the coagulating tank is about 5.3 to 5.8.
 7. The process of claim 1 in which the gel structure is stirred by moving a perforated partition plate upward in the coagulating tank while causing the gel structure to pass downward through the perforations in said perforated plate.
 8. The process of claim 1 in which the gel structure is stirred by temporarily interrupting the downward flow of the fermenting mixture with a partition plate and stirring the gel structure below said partition plate.
 9. A process for the continuous preparation of Bulgarian yoghurt comprising adding a continuous stream of milk to a fermenting mixture consisting of milk and starter in a prefermenting tank while maintaining the temperature of the mixture at 42*-48*C, transferring a similar quantity of the fermenting mixture which has a pH not below about 5.3 into a coagulating tank in which further fermentation and coagulation take place, cooling the fermenting mixture during passage into the coagulating tank, spraying the fermenting mixture into the coagulating tank, allowing the fermenting mixture to flow downward through the coagulating tank and coagulate and form a gel structure, and near the bottom of the tank where the acidity is sufficient to produce a pH of the gel structure of about 5.1 or below and the acidity is sufficiently high to prevent syneresis by stirring, stirring the gel structure near the bottom of the tank to form a smooth consistency without disturbing that part of the gel structure upstream in the coagulating tank in which sufficient acidity has not been produced, and subsequently filling containers with the resulting product through a discharge pipe in the bottom of the coagulating tank.
 10. A process according to claim 9 in which the coagulating tank walls are coated with a surfaceactive compound.
 11. A process according to claim 9 in which the speed of the fermenting mixture in the coagulating tank is at most 5 cm/minute at a temperature of 37*C.
 12. A process according to claim 9 in which the temperature at which the fermenting mixture is transferred into the coagulating tank is held constant.
 13. The process of claim 9 in which the temperature near the bottom of the tank is about 33* to 37*C.
 14. The process of claim 9 in which the pH of the fermenting mixture transferred into the coagulating tank is about 5.3 to 5.8.
 15. The process of claim 9 in which the gel structure is stirred by moving a perforated partition plate upward in the coagulating tank while causing the gel structure to pass downward through the perforations in said perforated plate.
 16. The process of claim 9 in which the gel structure is stirred by temporarily interrupting the downward flow of the fermenting mixture with a partition plate and stirring the gel structure below said partition plate.
 17. A process for the continuous preparation of cultured buttermilk or lactic starter comprising adding a continuous stream of milk to a fermenting mixture consisting of milk and starter in a pre-fermenting tank while maintaining the temperature of the mixture at 20*-35*C, transferring a similar quantity of the fermenting mixture which has a pH not below about 5.3 into a coagulating tank in which further fermentation and coagulation take place, spraying the fermenting mixture into the coagulating tank, allowing the fermenting mixture to flow downward through the coagulating tank and coagulate and form a gel structure, and near the bottom of the tank where the acidity is sufficient to produce a pH oF the gel structure of about 4.7 or below and the acidity is sufficiently high to prevent syneresis by stirring, stirring the gel structure near the bottom of the tank to form a smooth consistency without disturbing that part of the gel structure upstream in the coagulating tank in which sufficient acidity has not been produced, and subsequently filling containers with the resulting product through a discharge pipe in the bottom of the coagulating tank.
 18. A process according to claim 17 in which the coagulating tank walls are coated with a surface active compound.
 19. A process according to claim 17 in which the speed of the fermenting mixture in the coagulating tank is at most 5 cm/minute at a temperature of 35*C.
 20. A process according to claim 17 in which the temperature at which the fermenting mixture is transferred into the coagulating tank is held constant.
 21. The process of claim 17 in which the pH of the fermenting mixture transferred into the coagulating tank is about 5.3 to 5.8.
 22. The process of claim 17 in which the gel structure is stirred by moving a perforated partition plate upward in the coagulating tank while causing the gel structure to pass downward through the perforations in said perforated plate.
 23. The process of claim 17 in which the gel structure is stirred by temporarily interrupting the downward flow of the fermenting mixture with a partition plate and stirring the gel structure below said partition plate. 